As is well known in the art, the quantity of carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx) emitted when gasoline fuel is combusted in a spark-ignited internal combustion engine is influenced predominantly by the air-to-fuel ratio in the combustion cylinder. An exhaust gas having a stoichiometrically balanced composition is one in which the concentrations of oxidising gases (NOx and O2) and reducing gases (HC and CO) are substantially matched. The air-to-fuel ratio that produces the stoichiometrically balanced exhaust gas composition is typically given as 14.7:1.
Theoretically, it should be possible to achieve complete conversion of O2, NOx, CO and HC in a stoichiometrically balanced exhaust gas composition to CO2, H2O and N2 and this is the duty of a so-called three-way catalyst. Ideally, therefore, the engine should be operated in such a way that the air-to-fuel ratio of the combustion mixture produces the stoichiometrically balanced exhaust gas composition.
Another way of defining the compositional balance between oxidising gases and reducing gases of the exhaust gas is the lambda (λ) value of the exhaust gas, defined according to equation (1) as:Actual engine air-to-fuel ratio/Stoichiometric engine air-to-fuel ratio,  (1)wherein a lambda value of 1 represents a stoichiometrically balanced (or stoichiometric) exhaust gas composition, wherein a lambda value of >1 represents an excess of O2 and NOx and the composition is described as “lean” and wherein a lambda value of <1 represents an excess of HC and CO and the composition is described as “rich”. It is also common in the art to refer to the air-to-fuel ratio at which the engine operates as “stoichiometric”, “lean” or “rich”, depending on the exhaust gas composition which the air-to-fuel ratio generates: hence stoichiometrically-operated gasoline engine or lean-burn gasoline engine.
It should be appreciated that the reduction of NOx to N2 using a TWC is less efficient when the exhaust gas composition is lean of stoichiometric. Equally, the TWC is less able to oxidise CO and HC when the exhaust gas composition is rich. The challenge, therefore, is to maintain the composition of the exhaust gas flowing into the TWC at as close to the stoichiometric composition as possible.
Of course, when the engine is in steady state it is relatively easy to ensure that the air-to-fuel ratio is stoichiometric. However, when the engine is used to propel a vehicle, the quantity of fuel required changes transiently depending upon the load demand placed on the engine by the driver. This makes controlling the air-to-fuel ratio so that a stoichiometric exhaust gas is generated for three-way conversion particularly difficult. In practice, the air-to-fuel ratio is controlled by an engine control unit, which receives information about the exhaust gas composition from an exhaust gas oxygen (EGO) (or lambda) sensor: a so-called closed loop feedback system. A feature of such a system is that the air-to-fuel ratio oscillates (or perturbates) between slightly rich of the stoichiometric (or control set) point and slightly lean, because there is a time lag associated with adjusting air-to-fuel ratio. This perturbation is characterised by the amplitude of the air-to-fuel ratio and the response frequency (Hz).
The active components in a typical TWC comprise one or both of platinum and palladium and rhodium supported on a high surface area oxide.
When the exhaust gas composition is slightly rich of the set point, there is a need for a small amount of oxygen to consume the unreacted CO and HC, i.e. to make the reaction more stoichiometric. Conversely, when the exhaust gas goes slightly lean, the excess oxygen needs to be consumed. This was achieved by the development of the oxygen storage component which liberates or absorbs oxygen during the perturbations. The most commonly used oxygen storage component (OSC) in modern TWCs is cerium oxide (CeO2) or a mixed oxide containing cerium, e.g. a Ce/Zr mixed oxide.
A typical sensor arrangement for a modern TWC is to dispose a first lambda sensor for contacting exhaust gas on an upstream side of the TWC and a second lambda sensor for contacting exhaust gas on a downstream side of the TWC, i.e. to contact exhaust gas leaving the TWC. The first sensor is used to control the air-to-fuel ratio of the engine by closed loop control by inputting the sensor reading to an engine control unit. Principally, the second sensor is used for two purposes: to “trim” the control of the air-to-fuel ratio of the engine, which is the primarily the purpose of the first lambda sensor; and for use in on board diagnostics.
On board diagnostic systems are used continuously to monitor vehicular exhaust gas aftertreatment devices, such as TWCs, in order to report and set fault codes or alarm signals when the devices no longer meet prescribed emission standards.
Lambda sensors are expensive and it has been suggested recently to remove one lambda sensor and run the system on a single lambda sensor disposed within or immediately downstream of the TWC (see for example WO 2005/064139, the entire contents of which is incorporated herein by reference). Not only can this make the system overall less costly, but it is believed that, by locating the single lambda probe more intimately with the TWC, it is possible to reduce the time lag associated with adjusting air-to-fuel ratio, to control the lambda value of the exhaust gas more accurately and thereby increase the conversion efficiency. It may even be possible to use smaller TWCs comprising less of the expensive precious metal active components.